Typically, vacuum cleaners use a vacuum cleaner head having a dirty air inlet which is in flow communication with a source of suction (e.g. a motor driven fan unit). When the vacuum cleaner is turned on, the suction source creates a low pressure area which draws air into the dirty air inlet. Dirt particles and the like are entrained in the air flow and transported by the air flow to a dirt separation mechanism provided with the vacuum cleaner. In order to assist the entrainment of dirt particles and the like in the air stream entering the dirty air inlet, an agitator (e.g. a rotatably mounted brush) is provided. The rotation of the brush agitates the surface (e.g. carpet) over which the vacuum cleaner head travels. This agitation disturbs the dirt which is in the carpet so that it may more easily be entrained in the air entering the dirty air inlet.
It has also been known in the vacuum cleaning art to include a height adjustment mechanism so that the position of the rotatably mounted brush with respect to the dirty air inlet may be adjusted to position the brush for optimal contact between the brush and the surface being cleaned. In such devices, the brush is mounted within the casing above the dirty air inlet and as the position of the brush is adjusted, the distance from the periphery of the brush to the inner portion of the casing housing the brush varies.
One disadvantage of this approach is that the air gap between the brush and the inner portion of the casing housing the brush varies. Accordingly, even if the portion of the casing housing the brush were aerodynamically designed so as to assist in the travel of the dirty air past the brush, the benefit of the aerodynamic shape would be reduced as the height of the brush is adjusted.